Corrosion resisting ferrous alloys



Patented Oct. 25, 1938 coanosron aasrsrme rsnaous anno s James A.Parsons, Jr., Dayton, Ohio, asslgnor to The Duriron Company, 1110.,Dayton, Ohio, a corporation of New York No Drawing. Original applicationDecember 28,

1935, Serial No. 56,586. Divided and this application September 20,1938, Serial No. 230,889

6 Claims.

This invention relates to corrosion-resisting ferrous alloys; and itrelates more particularly to ferrous alloys or alloy steelscharacterized by a relatively high content of nickel and chromium,

together with a further content of supplementing or fortifyingconstituents comprising copper, molybdenum and silicon in carefullyadjusted but somewhat variable percentages that are comparatively small,molybdenum being always present in substantially greater proportion thansilicon. The new alloys are characterized by a high degree of resistanceto corrosion and, in addition, by the fact that sound castings can beproduced therefrom and that they may also be forged and rolled hot withfar less loss than has been unavoidable with alloy steels of generallycomparable type heretofore known.

The present application is a division of prior I copending application,Serial No. 56,586, filed December 28, 1935.

Corrosion-resistant austenitic alloy steels of high nickel-chromiumcontent, fortified or reinforced with small amounts of silicon, copperand molybdenum, have been heretofore known and used for certainpurposes. But the range of theirpractical utility has been greatlyrestricted because they possessed certain inherent troublesomecharacteristics rendering them unsuitable for certain purposes or theirfabrication into required products very expensive. has been verydiflicult to produce from such alloys satisfactory sound castings, suchas valve and pump parts, that must be tight against hydrostaticpressure. Furthermore, in hot forging and rolling ingots of such prioralloy steels into bars, rods and sheets, for instance, the yieldsobtainable in practice seldom exceeded 50 per cent and were commonlylower, due to excessive but unavoidable waste, thus greatly increasingthe cost of the forged or rolled products and naturally limiting theiruse to special situations where such high cost would not be prohibitive.

It has now been found that the foregoing objections and disadvantagescharacterizing such prior alloy steels can be largely overcome byproperly adjusting and controlling the relative proportions and totalamounts of the before-mentioned reinforcing or fortifying constituents,that is, silicon, copper and molybdenum, in the manner hereinafter setforth; and that when this is done, the mechanical and physicalcharacteristics of the resultant alloy steels are greatly improved.Furthermore, their corrosion-resistance is not only fully as good asthat characterizing prior alloy steels of this general type but, in thebest For example, it

embodiment of the invention, is substantially enhanced.

The percentages of nickel and chromium characterizing ferrous alloys ofthe present invention, although capable of fairly wide variation, arerelativelyv large in all cases. The novel alloys with which the presentapplication is particularly concerned and which have especiallyoutstanding advantages and utility in the practical art, never containsubstantially less than 20 per cent nickel and 17 per cent chromium asmlnima. The nickel content may run as high as 23 per cent and thechromium content as high as 22 per cent, nickel always exceedingchromium in the alloy composition. Best results are obtained when theproportion of nickel is between 21 and 22 per cent, and that of chromiumis between 18 and 21 per cent. Iron, the largest single component,always constitutes at least about per cent of the alloy and usually morethan per cent.

Although the marked effect of copper in high nickel-chromium steels as afortifying agent to increase corrosion resistance has long been.recognized, its employment for this purpose, even in very smallproportions, has been attended heretofore with serious difficulties andobjections, especially because of its adverse effect upon thehot-working properties of such alloy steels through a tendency toproduce hot shortness.

According to the present invention, the fortifying or reinforcingelements are so proportioned that the highly desirable effect of copperin increasing corrosion-resistance can be utilized fully while at thesame time eliminating or sufficiently reducing its tendency to producehot shortness. It has been found that, in order to accomplish this, itis vital to have the percentage of molybdenum always substantiallygreater than that of silicon, and most desirably at least two or moretimes as great. The percentage of silicon should not be substantiallyless than 0.50, and as much as 2 per cent is permissible in practice. Itis generally very much better, however, to keep the silicon well below 2per cent, and indeed not to let it greatly exceed 1 per cent. For bestresults, the molybdenum content of the new alloy steels should be in theneighborhood of 3 to 3.5 per cent, ranging normally in practice from 2.0to 4.0 per cent. It is very important that the total or combinedpercentage of molybdenum and silicon content shall not substantiallyexceed 5 per cent, an upper limit of 4.5 per cent being ordinarilydistinctly preferable to observe. A higher aggregate percentage of theseconstituents is found to have a pronounced unfavorable effect upon thehot working properties of the alloy.

Alloy steels of the present invention are also characterized essentiallyby low carbon content not substantially exceeding 0.12 per cent. Indeed,it is only in exceptional cases that it is advisable to permit thecarbon content to exceed 0.10 per cent. Ordinarily it is found thatabout 0.06 to 0.08 per cent carbon is extremely satisfactory in mostcases, and a range of 0.04 to 0.07 per cent may be regarded as optimum.Too high a carbon content is to be avoided because it increasesvulnerability of the alloy to intercrystalline corrosion, especiallyupon exposure to contact with sulphuric acid.

It will be understood that the new ferrous alloys may contain manganesein variable percentages, e. g. 0.40 to 0.75 per cent; also minutequantities of the common impurities such as phos phorus and sulphurwhich, together, do not amount to more than 0.04 per cent or thereaboutsin the type of iron suitable to use in manufacturing alloy steels of thetype here in question.

By maintaining the ratio of molybdenum to silicon such that themolybdenum always substantially predominates, with their combinedpercentages not substantially exceeding 4.5 per cent, and by observingthe other precautions herein set forth, it becomes possible to employ,for example, from 1 to 3 per cent of copper in an alloy steelcontaining, say, 18 to 20 per cent chromium and from 22 to 23 per centnickel, which is of particularly great industrial value not only becauseof its high degree of corrosion resistance to sulphuric acid and otherdrastic corroding agents over a wide range of practical conditions, butalso because sound, tight castings can be made from, it and,furthermore, it can be easily forged and rolled with remarkably highyields of desired products. As a rule, such an alloy steel contains atleast 50 per cent iron.

Heretofore, in order to produce an alloy steelof this general typegiving a for ng y eld of even 50 per cent, it was necessary to keep thecopper content down to about 0.3 per cent as a maximum. The ability touse a much higher copper content in such alloy steels and thus toenhance materially their corrosion resistance, while obtaining not onlyas good but far higher forging" and rolling yields, is of the greatestimportance industrially.

In order to illustrate further the underlying principles of theinvention, and without intending thereby to limit its scope, thecompositions or analyses of a number of the new alloy steels that aretypical and representative are given in the following table showingpercentages of the essential components other than iron:

Example No. Ni Cr 8i Cu Mo 2). 0 19. 0 0. 1. 0 3. 5 0.07 (QM-0m). 22 021. 0 0. 5 0. 5 3. 5 20. 0 18. 0 l. 0 1. 0 3. 5

21. 0 18. 0 1. 0 1. 0 3. 0 22. 0 21. 0 1. 0 0. 5 3. 5 Z). 0 17. 0 1. 5l. 0 3. 5 20. 0 l7. 0 l. 5 0. 5 3. 5 R 0 22. 0 1. 0 1. 0 2. 0 Z). 0 18.0 0. 5 1. 0 4. 0 21.6 20.3 0.5 0.9 3.4 0.07 21. 0 20. 8 0. 5 1. 0 3. 50.12 at 8 l8. 5- 1. 0 O. 0 3. 2 0.06 21. 5 17. 8 1. 5 1. 0 3. 1 0.07 21.5 17. 5 l. 4 0. 9 3. 0 01B 21. 1 17. 4 l. 5 0. 9 3. 2 0.00 21. 6 18.l 1. 0 1. 0 2. 0 0.06 21. 5 l8. 1 l. 3 2. 8 3. 4 0.07

Alloy steels of the character represented by the foregoing analyses areall characterized by the fact that the reinforcing elements relied uponto give increased corrosion resistance as compared to ordinary highchromium-nickel steels are carried in a uniform, stable, solid solutioneven at ordinary temperatures. When subjected to drastic tests with theusual corroding reagents under conditions ranging from reducing throughneutral to strongly oxidizing, and at widely varying temperatures, thenew alloy steels show extremely high corrosion resistance, notably sotoward sulphuric acid. Their mechanical properties are also excellent.Whereas standard bend test specimens of prior alloy steels of generallycomparable type fail at less than 90 when bent cold, similar testspecimens of the new alloy steels show no cracking or other failure whenbent 90 nor, in many cases, even when bent 180. This is evidently due tothe superior microstructure of the new alloy steels which is found uponexamination to be homogeneous in character, there being little or noprecipitation of ferrite on the crystallographic planes orintracrystalline faces. Also there is practically no formation ofinsoluble carbides at the grain boundaries; and accordingly these newalloy steels are especially free from attack through intergranularcorrosion. Thin sections of castings produced from the new alloys aremuch less or not at all subject to the severe coring or double walleffect, commonly characterizing castings from comparable prior alloysteels, which coring is not .only conducive to porosity (leaky castings)but lowers corrosion resistance as well as the elastic limit of themetal.

Alloy steels having analyses approximately as given in the table givenhereinabove are regarded as particularly advantageous embodiments of theinvention because combining very high corrosion resistance withexceptionally good casting and hot working characteristics. For example,in actual mill operation with an alloy steel having substantially theanalysis given in Example No. 10, the yield from poured ingot tofinished rolled bar was 73 per cent in a typical instance. contrastswith a maximum obtainable yield of only about 50 per cent when rollingprior alloy steels of most nearly comparable type. Similar markedincreases in forging and rolling yields are characteristic of the newalloy steels generally.

The new alloy steels may be manufactured by ordinary methods well knownto those skilled in the art of making high nickel-chromium alloy steels,no special expedlents or precautions being required. 80 also, productionof castings, ingots, bars, sheets, etc., from these steels, and the heattreatment thereof, may be carried out in the usual way. It itaccordingly unnecessary to describe these methods here.

The term wrought as employed in certain of This the claims hereinafteris' to be understood as in its usual generic sense to include workingmetal as by forging or rolling, for example.

What is claimed is:

1. An alloy comprising the following elements in proportions fallingwithin ranges of percentages substantially as follows:

substantially all the balance to make 100 per cent being iron, which isnever less than per cent, molybdenum always being in excess of siliconand the sum of molybdenum and silicon never exceeding 5 per cent, nickelalways being in excess of chromium, said alloy being corrosion resistantand possessing good hot working properties, a standard test specimen ofsaid alloy being capable of withstanding a 90 cold bend without failure.

2. An alloy comprising iron, at least per cent; chromium 18 to 21 percent; nickel 21 to 22 per cent and predominating over the chromium;carbon, not exceeding 0.10 per cent; copper, 0.75 to 1.25 per cent;molybdenum, 3 to 3.5 per cent; and silicon 0.5 to 1.5 per cent, saidalloy being corrosion resistant and possessing relatively good hotworking properties, a standard test specimen of said alloy being capableof withstanding a cold bend without failure.

3. An alloy steel having approximately the following percentagecomposition: nickel 21.6, chromium 20.3, silicon 0.5, copper 0.9,molybdenum 3.4, and carbon 0.07, substantially all the remainder beingiron, said alloy being corrosion resistant and possessing relativelygood hot working properties, a standard test specimen of said alloybeing capable of withstanding a 90 cold bend without failure.

4. An alloy steel having approximately the following percentagecomposition: nickel 21, chromium 18, copper 1, silicon 1, molybdenum 3,and carbon 0.07; practically all the remainder being iron, said alloybeing corrosion resistant and .possessing relatively good hot workingproperties, a

35 standard test specimen of said alloy being capable of withstanding a90' cold bend without failure.

5. A corrosion resistant wrought ferrous metal article having thefollowing elements in proportions falling within ranges of percentagessubstantially as follows:

Per cent Nickel 20.00 to 23.00 Chromium 17.00 to 22.00 Silicon 0.50 to1.60 Copper 0.50 to 2.80 Moly n 2.00 to 4.00 Carbon up to 0.12

Per cent Nickel 20.8 Chromium 18.5 Silicon- 1.0 Copper 0.9 Molybdenum3.2 Carbon 0.08

the balance to make per cent being substantially all iron.

' JAMES A. PARSONS, Jn.

